Neutron radiography is a non-destructive testing process used for determining certain material characteristics. In its simplest terms, neutron radiography is accomplished by irradiating an object with neutrons and measuring the resulting flux of neutrons which pass through the object per unit area. Material variations and characteristics of the tested object may thereby be detected by observing variations in neutron flux through the object. Neutron radiography is most often accomplished using thermal neutrons, or those neutrons having an average kinetic energy of about twenty-five one thousandths electron volts (0.025 eV). Some applications of neutron radiography require the use of cold neutrons, or neutrons having a kinetic energy less than five one thousandths electron volts (0.005 eV). Other applications may require the use of epithermal neutrons, or neutrons having kinetic energies up to a few electron volts. Neutrons with any of these energy levels are usually produced by thermalizing, or "moderating", high energy neutrons (up to several MeV), known as fast neutrons, which are emitted by a neutron source. This moderation is accomplished by directing the fast neutrons through a moderating material. The thermalization process results in an energy spectrum of neutrons essentially in thermal equilibrium with the moderating material.
Several different types of fast neutron sources for radiography are used. For example, nuclear reactors are one source of neutrons which provide highly intense beams of neutrons for irradiating an object to be tested. For many important applications of neutron radiography, however, it is desirable to have a neutron source other than a nuclear reactor.
An alternative source of neutrons known in the prior art is a Van de Graff accelerator. It produces neutrons for irradiation by accelerating deuterons against a beryllium target to produce fast neutrons. The beryllium target is disposed in a chamber of moderator material appropriate for moderating, or thermalizing, the fast neutrons down to the desired neutron energy level for radiography. An example of such a device is discussed in U.S. Pat. No. 4,599,515 which issued to Whittemore.
Yet other sources of neutrons for radiography are radioisotopes. One such radioisotope is californium-252, which emits, by radioactive decay, approximately 10.sup.6 neutrons per second per microgram of californium. The seemingly large neutron flux produced by californium-252 and other radioisotopes, however, is still much less intense than the neutron flux produced by a nuclear reactor. It is, therefore, important to make optimum use of all available neutrons when using a radioisotope neutron source in order to obtain a good image with a minimum of exposure time. A neutron radiography device using a radioisotope as the neutron source may incorporate one or several design features to fulfill the requirement of optimum use of available neutrons. For example, as is well known in the art, neutron flux will vary throughout the radiography device. The radiography apparatus could be designed to establish a maximum thermal neutron flux at the imaging plane. One way of achieving this result is to dispose the radioisotope sources in a neutron moderator such that the imaging plane is exposed to the maximum thermal neutron flux in the moderating material. A second method of making optimum use of available neutrons is to shield the imaging plane from non-neutron radiation "noise". Such noise primarily consists of electromagnetic radiation, such as gamma rays produced by source radioisotope decay. The gamma rays, by irradiating the imaging plane, can thereby lower system photographic contrast. In the past, conventional radioisotope neutron radiography devices have provided poor shielding of gamma rays emerging directly from the radioisotopes. This is due, in part, to some of these prior devices disposing the neutron source in line-of-sight of the imaging plane, in an attempt to obtain maximum neutron flux. Unfortunately, such line-of-sight placement also results in a substantial amount of direct gamma irradiation of the imaging plane.
A third design technique to optimize the number of available neutrons in a radiograph is to establish a uniform neutron flux across the imaging plane by opening up the full neutron source plane as an optimum source of neutron flux. By establishing a substantially uniform neutron flux across the entire imaging plane, a greater area of the test object per exposure may be radiographed. Conventional designs for neutron radiography devices which use radioisotopes as the neutron source tend to produce a neutron flux which is not uniform across a relatively large imaging plane. This, in turn, either reduces the effective area of the test object which can be radiographed with each exposure, or results in lower photographic image density of those parts of the imaged object or objects near the boundaries of the imaging plane.
Accordingly, it is an object of the present invention to provide a neutron radiography deice which optimizes neutron flux across the imaging plane. It is a further object of the present invention to provide a neutron radiography device which produces a uniform neutron flux across the imaging plane. Still another object of the present invention is to provide a neutron radiography device which accomplishes the above while substantially shielding gamma rays from the imaging plane. Another object of the present invention is to provide a neutron radiography device that is durable, reliable, and cost-effective in its manufacture and use.